This invention relates to a chemical method of strengthening glass articles subjected to an abrasion resistance treatment and more particularly, it is concerned with a method of strengthening a glass article by chemically effecting exchange of a relatively larger radius ion, K.sup.+ ions, for a smaller radius ion, Na.sup.+ ions, in the surface layer of the interior or exterior of the glass article, which outer surface has been coated with a metal oxide, to thus form a compressive stress layer on the interior or exterior surface layer.
It is known in the prior art that the intrinsic strength of glass is very high, for example, more than 7,000 Kg/cm.sup.2 in the non-scratched state, called "pristine glass", but, when the surface is bruised, the strength is substantially lowered, for example, to about 200 Kg/cm.sup.2. When glass articles are handled, for example, during inspecting, bottling, capping, packaging and shipment, the glass articles are brought into contact or friction with each other and thus scratched or bruised, whereby their strength is lowered to a great extent.
Various proposals have hitherto been made in order to protect glass articles from scratching or bruising and to improve the strength thereof. For example, there has been proposed a method of increasing chemically the mechanical strength of glass by the so-called ion exchange method wherein ions A contained in the glass surface, such as sodium ions, are replaced with ions B having a larger ion radius, such as potassium ions. This chemical strengthening treatment is ordinarily carried out by the so-called immersion method comprising contacting a glass article with a potassium salt bath at an elevated temperature below the strain point to effect exchange of potassium ions for sodium ions, or by the so-called coating method comprising spraying a potassium salt solution onto a glass article or dipping a glass article in a potassium salt solution, while holding the glass article at an elevated temperature near the strain point, and holding the thus coated glass article at this temperature for a period of time sufficient to effect the ion exchange to thus produce a compressive stress layer. These methods are described in Japanese Patent Publication No. 28674/1965 (British Pat. No. 966,734), No. 6610/1973 (U.S. Pat. Nos. 3,498,773 and 3,473,906), No. 1316/1972 (U.S. Pat. No. 3,607,172) and No. 49298/1972 (British Pat. No. 1,010,164).
In Japanese Patent Publication No. 1307/1972 there is described a method of strengthening glass bodies by combining the above described ion exchange strengthening treatment with a metal oxide coating and polymer coating. This method involves treating the surface of a glass body at a high temperature with a metal compound to pyrolyze the metal compound immediately and to form the respective metal oxide film on the glass surface, then spraying the surface with an aqueous solution of potassium phosphate, maintaining the glass body at a temperature and for a period of time sufficient to effect the ion exchange reaction, cooling and washing and then spraying onto the glass surface an olefin polymer. In this method, a glass surface having a temperature of at least 530.degree. C., immediately after being formed, is sprayed with a solution for ion exchange treatment. This method can be applied to light weight and thin glass articles but cannot be satisfactorily applied to thick glass articles, which must be treated at a temperature below the strain point. Furthermore, this method has the disadvantage that the ion exchange of the inner surface of a glass article is technically impossible because the glass article is treated at a high temperature as described above and there is therefore a danger of breakage of the glass article, and complex apparatus is required for the strengthening treatment of the inner surface of the glass article. Therefore, glass articles, in particular returnable bottles which are recovered and reused, tend to be scratched or bruised, during washing, on the inner surface thereof and are often broken. Also, according to the immersion method, the ion exchange of the inner surface of a high temperature glass article immediately after being formed cannot be easily effected because of the danger of breakage, and usually can only be effected by inserting in the glass article a nozzle through which a treating solution is fed. In this method, however, it is also necessary to hold the glass article at a high temperature so that it is not deformed and to use a heat resisting holder for holding the glass article, which can resist sufficiently high temperatures. Similarly a heat resisting material which can resist sufficiently high temperatures must be used for the nozzle to be inserted in the glass article. Unless centering of the above described nozzle is suitably carried out, the nozzle is contacted with the opening portion of the glass article when it is inserted to thus deform or break the opening portion and thus lower the value of the glass article. In order to solve this problem, a complex centering technique is required. In the case of a thick glass article, furthermore, there arise other troublesome problems, for example, breakage due to thermal shock, since the treatment of the glass surface at an elevated temperature below the strain point followed by quenching results in a temperature difference between the surface and inside due to a low heat conductivity of glass, so that the surface contracts more than the inside, and a tensile stress is produced on the surface and a compressive stress is produced on the inside.
When a glass article is shocked on the outer surface, a tensile stress is produced on the inner surface and the glass article is often broken. Therefore, it is necessary for practical use to strengthen both the outer and inner surfaces of a glass article. We have already proposed a method for strengthening both surfaces of glass articles in Japanese Patent Application No. 136182/1973. It will be understood from this prior invention that a beer bottle (Special Container Form: Form No. 7, Volume 633 ml) whose outer and inner surfaces are chemically strengthened is very superior in pressure resistance and shock resistance to a beer bottle whose outer surface only is chemically strengthened. That is to say, the additional chemical strengthening treatment of the inner surface of a glass container can produce unexpected results.
As a surface treating agent for the ion exchange strengthening treatment of glass articles various potassium salts have been used, for example, potassium nitrate (MP = 330.degree. C.). potassium chloride (MP = 790.degree. C.). potassium sulfate (MP = 1069.degree. C.), potassium carbonate (MP = 891.degree. C.), tribasic potassium phosphate (MP = 1340.degree. C.), potassium metaphosphate (MP = 807.degree. C.), potassium bromide (MP = 730.degree. C.) and potassium iodide (MP = 723.degree. C.). Some of these compounds tend to be hydrolyzed in aqueous solutions when applied to glass surfaces by spraying or dipping, thereby giving alkalinity and etching the glass surface. In the case of potassium carbonate, for example, potassium hydroxide formed by hydrolysis thereof etches glass. Tribasic potassium phosphate, potassium metaphosphate, potassium iodide and potassium bromide also have a similar tendency.
However, three potassium salts, potassium nitrate, potassium chloride and potassium sulfate, can favourably be used without the above described disadvantage, since even if these salts are hydrolyzed their hydrolyzed products are neutral and do not etch glass, thus the appearance of the treated glass remains transparent. We have now found, as a result of various experiments on the ion exchange strengthening treatment of a glass article having a metal oxide layer on the surface thereof using a mixed solution of potassium nitrate, potassium chloride and potassium sulfate, that a compressive stress layer and a sufficient compressive stress value can be realized by controlling, in particular, the thickness of the metal oxide layer to be initially applied, the adhesion time or temperature of the ion exchange solution and the mixing ratio of the above described three potassium salts.